1. Field of the Invention
The invention relates to an air-fuel ratio imbalance detecting device and air-fuel ratio imbalance detecting method for an internal combustion engine of a vehicle.
2. Description of Related Art
In an existing art, there is known a device in which, in a state where an operating state of an internal combustion engine is a predetermined steady operation state, when a variation per unit time in air-fuel ratio detected by an air-fuel ratio sensor provided in an exhaust pipe falls outside a predetermined range, it is determined that there is an imbalance in air-fuel ratio among cylinders of the internal combustion engine (for example, see Japanese Patent Application Publication No. 2011-144785 (JP 2011-144785 A)). In this way, the device described in JP 2011-144785 A simply determines whether there is an imbalance in air-fuel ratio among the cylinders of the internal combustion engine.
In the above-described existing air-fuel ratio imbalance detecting device for an internal combustion engine, when an intake air amount of the internal combustion engine is small, an output value of the air-fuel ratio sensor becomes small. This reduces a difference between the output value when there is an imbalance in air-fuel ratio among the cylinders of the internal combustion engine and the output value when there is no imbalance.
Therefore, the existing air-fuel ratio imbalance detecting device for an internal combustion engine may not be able to detect an imbalance in air-fuel ratio among the cylinders of the internal combustion engine or may erroneously detect an imbalance.